The invention relates to a display device comprising a cathode ray tube and a deflection unit, the cathode ray tube including an in-line electron gun having a main lens portion with means to generate a main lens field and a quadrupole field, and the display device comprising means to vary dynamically the strength of the main lens field and of the quadrupole field.
The invention also relates to a cathode ray tube which is suitable for use in a display device.
Display devices are used in, inter alia, television receivers and colour monitors.
In operation, the deflection unit generates an electromagnetic field for deflecting electron beams generated by the in-line electron gun across the display screen. The deflection field has a defocusing effect on the electron beams and causes astigmatism. Said effects vary with the degree of deflection. The electron gun comprises means to generate a main lens field and a quadrupole field and the display device comprises means to vary dynamically the strength of the main lens field and of the quadrupole field. This enables astigmatism and focusing of the electron beams as a function of deflection to be controlled so that astigmatism caused by the deflection field is at least partly compensated and that the electron beams are substantially in focus everywhere on the display screen. This improves the reproduction of the picture. Such electron guns are sometimes referred to in literature as DAF-guns (Dynamic-Astigmatism and Focusing).